1. Field of the Invention
The present invention relates generally to image capturing apparatuses, image capturing methods, and programs.
More particularly, the present invention relates to an image capturing apparatus capable of performing an auto focus operation using invisible light in addition to visible light, an image capturing method, and a program.
2. Description of the Related Art
Recently, digital cameras or video cameras having an auto focus function for automatically adjusting the focus have been widely used.
A contrast detection method has been suggested as one of auto focusing methods. In the contrast detection method, contrast values are calculated on the basis of luminance values of image signals resulting from image capturing performed while moving an image capturing lens. A subject is determined to be in focus at a highest contrast value. The image capturing lens is driven to move to a position corresponding to the highest contrast value. Since a focused state is determined using captured image signals in an auto focus (AF) function according to such a contrast detection method (hereinafter, referred to as “contrast-detection-based AF function”), the accuracy of the focusing is high and is not affected by aging of mechanical devices. In addition, the contrast detection method advantageously reduces the cost since it does not require mechanical operations for focusing. However, in the contrast detection method, focusing of the lens on moving subjects is difficult since the method takes some time for focusing. Additionally, focusing of the lens on low-contrast subjects is also difficult.
Accordingly, single lens reflex (SLR) digital cameras, for which higher continuous shooting performance and higher moving subject focusing performance are required, employ an AF function according to a phase difference detection method (hereinafter, referred to as “phase-difference-detection-based AF function”). The phase difference detection method advantageously allows a focal point to be rapidly determined since a distance is directly measured. However, since the distance is measured with an apparatus independent from that for image capturing, the method requires another mechanical device for automatic focus detection, which undesirably increases the cost, causes the parallax, or decreases the focusing accuracy due to aging of the mechanical apparatus.
To overcome such disadvantages regarding the focusing time and the focusing accuracy, a hybrid AF function that utilizes both the phase-difference-detection-based and contrast-detection-based AF functions has been suggested. For example, the phase-difference-detection-based AF function is mainly used in a sport mode that requires higher continuous shooting performance and higher moving subject focusing performance, whereas the contrast-detection-based AF function is mainly used in a landscape mode and a macro mode. In addition, the focus is finely adjusted using the contrast-detection-based AF function after being roughly adjusted using the phase-difference-detection-based AF function.
Nevertheless, even in the hybrid AF method, a focusing capability undesirably decreases for low contrast subjects or under a low illumination environment. Accordingly, methods for irradiating auxiliary light onto a low-contrast subject or a subject under the low illumination environment to adjust the focus and detect a distance to the subject using the reflected light have been suggested. For example, a technique for irradiating strobe light as AF auxiliary light when the brightness is equal to or lower than a predetermined level has been suggested (see Japanese Patent No. 3139067). In addition, a technique for irradiating near-infrared light as AF auxiliary light has been suggested (Japanese Unexamined Patent Application Publication No. 05-196859).